While plastic materials derived from petroleum that is a finite resource have been heavily used, the recent spotlight is focused on environmental load-reducing technologies. With this tendency, materials using cellulose fibers, which are biomass abundantly occurring in nature, have been attracting attention, and various techniques about the improvement thereon have been proposed. For example, patent literatures 1 and 2 below describe a paper material using oxidized pulp obtained by treating pulp with an oxidizing agent in the presence of an N-oxyl compound, such as 2,2,6,6-tetramethyl-1-piperizin-N-oxyl (hereinafter abbreviated as TEMPO).
A composite material obtained by mixing fibers with a nanoscale diameter (nanofibers), called microfibrils, with other materials is known. For example, patent literature 3 below discloses a composition containing a thermoplastic polymer matrix and a cellulose filler formed of cellulose microfibrils with a diameter of about 2 to 30 nm.
Patent literature 4 below describes a degradable or biodegradable polymer composition containing a biodegradable plastic and ascidian cellulose fibers. The ascidian cellulose fibers are microfibers with a diameter of 0.1 μm or less obtained by cutting the epitheca of ascidians with, e.g., a cutter, into pieces of about 5 to 20 mm, crushing the pieces using a mixer or a like machine, followed by microfibrillation using a beater or a like machine.
Patent literature 5 below describes a high strength material having a bending strength in a specific range as measured by a prescribed method of measurement, which contains 65% to 100% by weight of cellulose microfibrils and 0% to 35% by weight of an additive (e.g., a thermosetting resin, a thermoplastic resin, or starch) each based on the solids content.
The assignee common to this application previously proposed a gas barrier composite molded article having a layer of a gas barrier material containing cellulose fibers having an average fiber diameter of 200 nm or smaller and a carboxyl group content of 0.1 to 2 mmol/g (see patent literature 6 below). The cellulose fibers used here have a smaller diameters than conventional microfibers called nanofibers and are obtained by oxidizing natural cellulose fibers, such as wood pulp, in the presence of a TEMPO catalyst and subjecting the resulting oxidation product dispersion to a defibration operation using a mixer or a like device. The gas barrier composite molded article of patent literature 6 exhibits high gas barrier performance against various gases, including oxygen, water vapor, carbon dioxide, and nitrogen.
Non-patent literatures 1 through 3 below report obtaining a polylactic acid/cellulose nanowhisker (needle-like cellulose microfiber) composite material from a dispersion of cellulose nanowhiskers in an organic solvent, e.g., toluene, cyclohexane, or chloroform.
Cellulose nanowhiskers are obtained by hydrolysis of the amorphous regions of a cellulose raw material with sulfuric acid, followed by ultrasonication. Although the cellulose nanowhiskers are stably dispersible in water owing to the sulfuric acid group introduced into the cellulose units as a result of hydrolysis, they usually agglomerate or flocculate without being dispersed stably in an organic solvent, such as an alcohol or a non-aqueous solvent. In order to obtain a composite material composed of a plastic material such as polylactic acid and cellulose nanofibers such as cellulose nanowhiskers and exhibiting the full characteristics of the cellulose nanofibers, it is necessary to stably disperse the cellulose nanofibers in an organic solvent in which the plastic material is soluble. In this regard, the techniques of the non-patent literatures 1 to 3 achieve stably dispersing cellulose nanowhiskers in an organic solvent by modifying (hydrophobilizing) cellulose nanowhiskers by treating with an anionic surfactant, such as a phenyl-containing phosphoric ester.
Patent literature 6 supra also proposes a gas barrier material containing cellulose fibers having an average fiber diameter of 200 nm or smaller and a carboxyl group content of 0.1 to 2 mmol/g. The cellulose fibers are microfibers having a smaller diameter than conventional microfibers called nanofibers and are obtained by oxidizing natural cellulose fibers, such as wood pulp, in the presence of a TEMPO catalyst and subjecting the resulting oxidation product dispersion to a defibration operation using a mixer or a like device.